High Frequency feedback with FRFR - A lesson in diagnosing the issue

Sleestak

Sleestak

Power User
There have been a number of threads on the forum describing the occurrence of high frequency / microphonic feedback with FRFR systems, and I've been noticing over the past six months that this is becoming a more regular issue for me. I want to outline what I have experienced, and offer some troubleshooting suggestions.
It turned out the source of my issues was neither the AxeFX or my FRFR cabs, but it took some sleuthing to isolate the cause.
  • High frequency feedback is more likely to occur with a full-range speaker system than with traditional guitar cabinets. Typical guitar amp speakers are single-driver 10" or 12" units with limited frequency response, especially at the top end. My Euphonic Audio VL110 cabinets, and my Xitone 1x12 wedge are full-range with greater high-frequency capabilities. So, the presence of those frequencies is something guitarists may not be accustomed to onstage.
  • Higher gain tones exacerbate the HF feedback issue. Greater drive typically produces additional upper partials to the fundamental frequencies.
  • Single coil pickups are naturally more susceptible to both RF noise, and to microphonic feedback - even when the pickup is operating normally. I use single-coil pickups in nearly all my guitars (strats / teles / LPs with P90s).
  • Microphonic pickups cause HF feedback, thin tone, and other anomalies. This is much worse with higher gain on the amp.
So, about six months ago I noticed that my main guitar was starting to feed back in a weird unmusical way, and I began to dial down my gain in an attempt to control it. I switched to my primary backup guitar, and although it wasn't as bad, it still was happening. My assumption was that I had isolated the guitar from the equation by trying a different instrument, but experiencing a similar result. Then I started to check my cables, my wireless, and everything else in the system, but nothing seemed to make a difference. I updated my firmware and rebuilt patches. No difference. I rolled off a bunch of high end. No difference. Eventually, my main guitar started to pick up stray radio signals (!) including audio from the PA itself. I instinctively felt something must be wrong with the guitar.

Once we were at home long enough for me to schedule time with my favorite guitar tech, I took both my main guitar and the backup guitar to him. We immediately diagnosed my primary guitar as being extremely microphonic. It sounded terrible, but I'd been able to get a usable sound out of it by aggressively tweaking the AxeFX to compensate. It has become so microphonic and unstable that it was acting as an antenna. So, that guitar's pickups are now (even as I post this), taking a wax jacuzzi in the shop. We then turned our attention to guitar #2, which turned out to be somewhat microphonic. Its pickups are also being repotted.

I had no reason to believe it was possible for both of those guitars to have the same issue. But then again... they both have traveled with me for years, and although they have different pickups in them, I did have them configured at about the same time, 25 years ago. All that travel, on buses, planes, and trucks, exposed the guitars to a lot of heat and vibration. Plus, after 25+ years, the wax in those pickups had probably gotten flaky and dry.

I used a different guitar for the gigs last weekend, and my rig sounded amazing! I went back and restored the normal curve to my patches, and the AxeFX sounded incredible. I had gotten accustomed to my tone becoming progressively weaker and more thin, and hadn't noticed it was actually a problem until it became unbearable, and then I thought back to earlier this spring when I first started to notice the occasional feedback problems.

I'll get my main guitars back this week, and I can't wait to hear them, restored to their proper sound.

My recommendation for a lesson in this is to check your assumptions, and if you haven't found the issue, check your assumptions again. If I had grabbed somebody else's guitar, or even just another one from home, I would have immediately been able to diagnose the issue. It sure seems unlikely, but in this case, a double-failure was occurring at the input source and I had a bit of scientific bias.
 
To add to that, standing right next to big, loud PA speaker will make the guitar scream because of the magnetic field from the tweeter. Or that's how I understood Cliff's explanation. Anyway, I got to experience that once on a gig. That's a really weird way of getting untolerable feedback howl, nothing like a normal nice musical feedback through the strings. Fortunately it's a weird scenario, but big speakers and small stage do meet sometimes.
 
Oh yes! That was one of my weird symptoms. I couldn't figure out why I was hearing the vocalist coming out of my full range cabs. I was definitely picking up the PA. Most disconcerting. Except for how cool it would have been if our vocalist had actually been somehow trapped inside my speaker cabinet.... :)
 
I made the mistake of building my music room in the basement right beside the main electrical panel for the house. Doh! So I definitely have to deal with way more stray EMF than I would like.

Curiously. I have an Ibanez AK85, which is a full-sized, full hollow jazz box:

ibanez-ak85-358076.jpg


and it will not feedback no matter how loud I crank an amp. On the other hand, my Gretsch 6120 (with stock Gretsch Filtertrons), screams like a banshee at half the volume. A little odd, especially since the 6120 has a bridge post which the AK85 lacks, and I assumed this would have made the 6120 less likely to feedback.

It never occurred to me that the difference might be the pickups. I wonder if TV Jones pickups are better potted than the standard Gretsch pickups?
 
Did have the problem once, it did not reoccur since 2 years now and a few firmwares got by, also I rolled back a tiny bit my floor monitor volume.
 
The interaction with PA and monitoring systems is the MAJOR issue for me.

In our practice room I will use my rig - with identical set ups and guitars and have NO squeal/feedback issues whatsoever - BUT when I go to play a gig - using the House PA and Monitors - 9 times out of 10 I have issues with feedback and high pitched squeal - and often I either need to move my monitor to BEHIND me - and in extreme cases I have to avoid using some of my higher gain presets..........so something interacts with the guitar/FRFR system.
 
After getting my (de-demonized) guitars back, I played some shows this weekend. It's amazing how much better they sound. They high-pitched feedback is almost 100% tamed. It's still possible, for all the reasons we've discussed in this thread. One thing that seems to have a big effect on the feedback issue is the monitor placement. I generally have my FRFR speaker cabs behind me, and that seems to thwart the feedback. When I move my monitor to the front, it it more likely to cause feedback with higher gain patches.
The most reliable zero-feedback mechanism I've found is to simply use in-ears with no cabs onstage. I can't always do that, as some members of my bands rely on stage amps to hear the other instruments. But in one group, it's feasible, and makes a huge difference in the overall stage sound.
 
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